Providing inherently aging chromium-nickel stainless steel with different tempers



Patented Apr. 9, 1946 MIUM-NICKEL STAINLESS STEEL WITH i v DIFFERENT TEMPERS Ernest H. Wyche and Raymond Smith,

Pittsburgh; Pa.

No Drawing. Application June 21, 1944,

a Serial No. 541,464

comm (or. 148-215) This invention is a method of treating chromium-nickel stainless steel containing carbon and at least one of the stronger-than-chromium carbide formers, such as titanium, "columbium, tantalum, etc., with its components that are ferrite formers proportioned to those that are austenite formers to cause the production of stress-laden ferrite at low temperatures when the steel cools to room temperature from temperatures where it is completely austenitic. Usually the steel contains aluminum. As disclosed and claimed in the Wyche and Smith applicatiomflled October 8, 1941, Serial No. 414,194, of which this application is a continuation-impart, the stress-laden ferrite, of such a steel, is inherently aging, by heating above room temperature, to a higher hardness and strength. By varying the proportion between the ferrite formers and the austenite formers the steel may be made to contain more or less of this aging ferrite, with the balance austenite, but this means of controlling its response to aging has certain undesirable features.

Normally, the steel is made tocontain as much of the stress-laden ferrite as possible. This is desirable for obvious reasons. At the same time, however, it is desirable'to provide'the user of the steel with steel of diiferent tempers, since a steel that ages to maximum hardness and strength may not have adequate ductility for certain applications, or, in other words, may be of too high a temper. That is to say, different applications require a steel that can be aged to different hardnesses and strengths, with the attendant diflerences in ductility, it being understood that these With the foregoing in mind, the present inven-' tion comprises heating the described steel containing the stress-laden ferrite-to temperatures just above its austenitizing temperatures to produce the lowest tempen-to temperatures considerably higher to produce the highest temper and to intermediate temperatures to produce an intermediate temper, the time at which the steel is maintained at temperature being relatively unimportant excepting that the steel must be brought to the temperature selected, this heating properties are incompatible in the sense that maximum hardness and strength has minimum ductility, whereas maximum ductility attends minimum hardness and strength. Since the stressbeing followed by cooling to produce the stressladen; inherently aging ferrite. Thereafter, the same aging treatment will age the diflerent tempers to different hardness, strength and ductility, the lowest temper having the lowest hardness and strength with the most ductility and the highest temper having the highest hardness and strength with the lowest ductility, while the intermediate tempers will age to intermediate values.

More specifically, the described steel is. preferably produced with a composition comprising from .05 to .08% carbon, from 15 to 18% chromium, from 4 to 7% nickel, 2% plus 5 to 12 times the carbon content titanium, from ..03 to 1% aluminum and manganese about for 7% nickel or 1 to 2.5% for 5% nickel or 3 to 6% for 3% nickel,

'withthe balance mainly iron. This represents the steel as it is commercially. made, titanium being more readily available in'this country than the other stronger-than-chromium carbide formers. With steel of this general analysis, three tempers provide a practicable commercial temper range. Of these, the lowest .results when the steel containing the stress-ladenierrne is heated to about 1300 F; and is then cooled to reproduce the stress-laden ferrite, an intermediate temper results from heating it to1400 F. and cooling as laden ferrite hardens and strengthens through 40 described, while ahigh'temper results from heatthe aging phenomena, it may be underaged or overaged'to lower hardness and strength with attendant greater ductility as compared to the ductility obtained by aging to maximum hardness and strength. However, there are limits to the properties that can-be obtained by varying the;

aging treatment, such limitations being inherent to assemblies including parts where 'maximum' strength and hardness is desired regardless oi ductility loss and parts where greater ductility is ingit to 1600 F. and cooling to produce the described ferrite, these temperatures varying somewhat with the composition.

In the case'of a specific example ofthe steel containing .,07%" carbon, .60% manganese, 7% nickel, 17% chromium, 30% titanium, 20% alumium, and with the balance mainly iron, the varidesired even though hardness and strength must I ous treatments provided the following results in the case of hot rolled sheet:

' I Pounds Pounds Per cent Per cent per use yield elongall l ch strength ji i tion Hit-H300 F.+950 r.

133, 000 2 158, 000 16 H 1M Fri-950 F.

165, M 177, 000 12. 5 BREW" F.+950 F.

The abovevalues represent tension tests; taken transverse to the rolling direction, but directional yield strengths in'both tension and compression for a given heat treatment are very nearly the same, as contrasted to what is to be observed in the case of ordinary austenitic stainless steelv strengthened by cold work alone. It is to be understood that the 950 FJheating for one-half hour represents the aging treatment which, it will 7 be noted, is the same in all instances although- 10 producing diflerent strengths and ductilities as represented by tensile strength and elongation hinn b 1 ad uate, for exam 1e. Furthert er bon,'from'15'to 18% chromium, froma'to 7% more, if the steelto which the temper imparting treatment is applied has been excessively cold worked, it is necessary to anneal it at a high I enough temperature to remove thetcold-work strain, this being followed 'bycooling to produce ,thestre s-iauen ferrite to permit application r the temper adjusting heat treatment. Cold work up to a 20% reduction in the thickness of sheet does not usually-require this preliminary anneal,

and greater amounts of cold reduction may be tolerated if the anneal providing thevarious tem-j pers is high enough in temperature. The diiferent-tempers also have different properties'in their unaged condition, what are considered the lower tempers inthe aged condition being harder and the higher aged tempers being softer prior to,

aging. That is to say, the'austenitizing temperature producing the lowest aged temper gives the highest unaged-temperand that-which gives the highest-aged'temper provides the lowest unaged temper. "The intermediate ,aged temper is atv tended by an intermediate unaged temper. The heat treatment providing the different tempers functions as a solution anneal. When this solution anneal isrelatively low in temperature. thefoi'lowing cooling to produce the stress-laden ferrite results in the steel having-a michoscopically vis'ible structure exhibiting a lamellar con- "stituent. with the higher solution anneals' pro- ,viding the higher tempers there is less of this lamellar constituentmicroscopically visible in the.

1600 F., for instance, result in excessive scaling, and warping, and, in any event, produce such a high temper that the steel, when properly aged, has too little ductility for some applications.

We claim: 1. A method of controlling the temper of chroe i mium-nickel stainless steel containing carbon and 65 steelstructure. Therefore, microscopic examina- 'at least one of the. stronger-than-chromium carbide formers such as titanium, columbium, tan-"- talum, etc., with-its components other than said carbide formers that are ferriteformers proportioned to those that are austenite formers to cause the production of stress-laden ferrite with any remainder austenite at low temperatures when the steel cools to'room temperatures from an aus-- tenitizing temperature, saidn method comprising heating the steel to temperatures just above said austenitizingtemperatures to produce the highest unaged andlowest aged temper, to temperatures considerably higher to produce the lowest unaged and highest aged temper, and to intermediate temperatures to produce an intermediate temper, followed by cooling to produce said ferrite;

' 2. A'- method of controlling the temper of age hardening steel containing from .05 to .08% carnickel, '.2% plus 5 to 12 times the carbonjcontent titanium, from .03 to 1% aluminum and manganese about for 7% nickel or, 1 to 2.5 for 5% nickel or 3 to 6% for 3% nickel, with the balance mainly iron; said method comprising heating the steel to about 1300 F. to give'it a higher stress-laden ferritio structure rendering it agehardening. V v r 3. A method of'controlling the temper of age hardening steel containing about '.07%"'carbon, manganese, 7% nickel, 17 chromium, .60% titanium, .20% aluminum, and with th'e'balance mainly iron; said method comprising heating the steel to about 1300 F. to 'give'itfa higher unaged and lower aged temper, to about 1400 F. to give it a medium temper and to about 1600 F. to give it slower-unaged and higher aged temper, followed by cooling the steel to provide it with a stress-laden ferriticJstructure rendering it age hardening.

4. A method of controlling the'temper of chromium-nickel stainless steel containing carbon and at least one of the stronger-than-chromium carbide formers such. as titanium, coluinbium, tantalum, etc., with its components other than said carbide formers that are "ferrite formers proportioned to those that are austenite' formers to cause the production of stress-ladenferrite with any remainder austenite at low temperatures when the steel cools to room temperatures from an austenitizing temperature for the structure producing said ferrite, said method comprising heating the steel 'to temperatures just above said austenitizing temperatures to produce the lowest aged temper, to temperatures considerably higher to produce the highest aged temper and to inter- :rnediate'temperatures to produce an intermediate temper followed by cooling to produce said ferrite, the steel being thereafter artificially aged by heating.

- ERNEST H. WYCHE.

RAYMOND SMITH. 

